Caius knew Aylin well enough to devise a plan.
[To Kill A Demon Lord…] may not have dedicated a lot of time to Aylin but the little that had been explored, especially in regards to her infatuation with Caius, had allowed him, as a reader, to really get a read on her.
Clingy and Obsessive were the best words to describe her. At least at the start of the story. Caius in the original story could hardly shake off her advances even if he consistently ignored and rejected them.
After Lochxen though, Aylin showed she had a capacity for change and growth. Through narration and her decision not to take a partner if it wasn't Caius, it was shown that her infatuation never ceased. Still, she managed not to let it consume her completely and focused on other important things in her life. Mainly getting groomed to become the Matriarch of the Khione Family after her mother.
In any case, Caius was sure he could foster that infatuation.
